A cornerstone of several common therapies for human diseases is the surgical use of a vein as an arterial conduit. Surgeons frequently create arteriovenous fistulae (AVF), the preferred access for hemodialysis. However, the poor maturation and patency of AVF, requiring additional re-do procedures and surgery, reflects our imperfect understanding of the biology of venous remodeling that leads to successful venous adaptation to the arterial environment. This knowledge gap creates an unmet medical need for novel approaches to enhance venous adaptation and maturation, to increase successful use of venous conduits. The tyrosine kinase receptor Eph-B4 is an embryonic determinant of veins. Diminished Eph-B4 expression is associated with shear stress conditions with laminar-like flow such as occurs during vein graft adaptation in humans and mice. We present exciting new data that: 1) our innovative mouse model of AVF faithfully recapitulates human AVF maturation including the presence of disturbed shear stress and an approximately 1/3 failure rate; 2) in both humans and mice, Eph-B4 expression initially increases during AVF adaptation to the arterial environment; 3) Eph-B4 function is essential for AVF remodeling; 4) Eph-B4 tyrosine- 774 is a critical regulator of Eph-B4 phosphorylation and activation of downstream signaling in vitro; 5) manipulation of Eph-B4 function via delivery of wild type or mutant Eph-B4 in vivo alters AVF remodeling; and 6) Akt-1 knockout mice have reduced AVF remodeling and altered responses to Eph-B4 stimulation. Our data suggests that surgical placement of a vein into the arterial environment regulates Eph-B4 phosphorylation and/or expression that is critical for successful venous adaptation and AVF maturation. We hypothesize that altering Eph-B4 activity will improve venous adaptation to the arterial circulation, thereby improving AVF maturation. We will use our innovative in vivo model of AVF maturation, as well as use a bioreactor that can control and deliver hemodynamic loads to small diameter vessels and endothelial monolayers in vitro, to test our hypothesis with the following specific aims: Aim I: Determine optimal Eph-B4 manipulation and delivery to enhance venous adaptation to the arterial environment and improve AVF maturation. Aim II: Determine whether Akt function is a mechanism of Eph-B4-mediated AVF maturation. The work in this proposal will have lasting impact by establishing that Eph-B4 is a component of the mechanotransduction mechanism in veins. We will test our novel hypothesis that Eph-B4 activity, or lack thereof, defines venous phenotype and function. We use an innovative strategy, as well as innovative in vivo and in vitro models, to manipulate Eph-B4 signaling and optimize delivery in vivo to alter vessel identity and thereby improve AVF maturation.